1. Field of the Invention
The present invention relates to phosphors which can emit light of high luminance under low-velocity electron excitation, fluorescent compositions containing such phosphors, and low-velocity electron excited fluorescent display devices.
2. Description of the Prior Art
Zinc activated zinc oxide phosphors (ZnO:Zn) are well known as phosphors which can emit light of high luminance under excitation by low-velocity electron beams. When excited by low-velocity electron beams, the ZnO:Zn phosphors emit green-white light of high luminance, and they are practically used for fluorescent screens for low-velocity electron excited fluorescent display devices.
A low-velocity electron excited fluorescent display device (hereinafter referred to simply as a "fluorescent display device") comprises, basically, an anodic plate having a fluorescent screen on one side thereof and a cathode disposed face to face with the fluorescent screen in an evacuated tube, whereby the fluorescent screen on the anodic plate is excited by low-velocity electron beams (usually low-velocity electron beams at an acceleration voltage of at most 100 V) emitted from the cathode to emit light of high luminance. The fluorescent display devices having fluorescent screens composed of the above ZnO:Zn phosphors are widely used as display elements for table-top calculators, various meters and testers, etc.
In recent years, reflecting the expansion of the field of the application of the fluorescent display devices, a variety of emitting colors are desired for the fluorescent display devices. Various developments have been made on the phosphors which are capable of emitting light other than green light under low-velocity electron excitation. As a result, there have been developed some phosphors which are capable of emitting colored light other than green light under low-velocity electron excitation. Among them, there are (Zn.sub.1-x,Cd.sub.x)S phosphors represented by a specific example of the composition of (Zn.sub.1-x,Cd.sub.x)S:Ag.
For the preparation of such (Zn.sub.1-x,Cd.sub.x)S phosphors, it is known to employ NaCl, as disclosed by H. W. Leverenz in "An Introduction to Luminescence of Solids" pages 196-199, John Wiley & Sons, Inc., 1950, or in Japanese Unexamined Patent Publications No. 129480/1980 and No. 667381/1982.
Further, in the above-mentioned Japanese Unexamined Patent Publication No. 167381/1982, it is proposed to increase the Ag content in the phosphor in order to improve the luminance of a low-velocity electron excited fluorescent display device. On the other hand, Japanese Examined Patent Publication No. 33153/1984 proposes to use a fluorescent composition composed of a mixture of a phosphor and a conductive material such as In.sub.2 O.sub.3 having a small particle size in order to achieve the same object.
However, it is now desired to further improve the luminance of the light emitted by a fluorescent display device during its use under low-velocity electron excitation i.e. at a low voltage and low electric power, in connection with a recent tendency for color display in the fluorescent display devices and an expansion of the field in which fluorescent display devices are used.